A plumb bob must be attached exactly in the center of the tripod head.
Our surveying apparatus was a triangle and a plumb bob which we made ourselves.
He had a plumb bob in his hand and a spirit level, but he laid them down.
When the instrument was level atop these rings, the plumb bob was theoretically over the center line of the cannon.
A protractor may be used in the same manner by sighting along the top and using a plumb bob to record the angle.
The plumb bob is then dropped, a distance of 20 ft. measured from it on the road, and a mark made.
He thought that a plumb bob dropped through the long axis of the rocket would form a right angle with the surface of Arzachel.